The present invention relates to an anode active material containing tin (Sn), cobalt (Co), and carbon (C) and a battery using it.
In recent years, many portable electronic devices such as a combination camera (Videotape Recorder), a mobile phone, and a notebook personal computer have been introduced, and downsizing and weight saving of such devices have been made. Research and development for improving the energy density of the battery used as a portable power source for such electronic devices, in particular the secondary battery as a key device has been actively promoted. Specially, a nonaqueous electrolyte secondary battery (for example, lithium ion secondary battery) provides higher energy density compared to a lead battery or a nickel cadmium battery as the traditional aqueous electrolyte secondary battery. Therefore, improvement thereof has been considered in various fields.
As an anode material used for the lithium ion secondary battery, carbon materials such as non-graphitizable carbon and graphite, which show a relatively high capacity and good cycle characteristics, have been widely used. However, taking account of the demand for high capacity in these years, it is a task to obtain a higher capacity of the carbon material.
From such a background, a technique for attaining a high capacity with a carbon material by selecting a carbonized material and preparation conditions has been developed (for example, refer to Japanese Unexamined Patent Application Publication No. H08-315825). However, in the case that such a carbon material is used, the anode discharge potential relative to lithium (Li) is from 0.8 V to 1.0 V, and the battery discharge voltage when forming the battery becomes low, and therefore improvement is not expected in view of the battery energy density. Further, there are disadvantages that hysteresis is large in the shape of charge and discharge curve, and energy efficiency in each charge and discharge cycle is low.
Meanwhile, as a high capacity anode exceeding the carbon materials, researches on alloy materials applying the fact that certain metals are electrochemically alloyed with lithium, and the alloy is reversibly generated and decomposed have been promoted. For example, a high capacity anode using Li—Al alloy or Sn alloy has been developed, and further a high capacity anode made of Si alloy has been developed (for example, refer to U.S. Pat. No. 4,950,566).
However, there is a large disadvantage that Li—Al alloy, Sn alloy, or Si alloy is expanded and shrunk associated with charge and discharge, and the anode is pulverized every charge and discharge, and therefore cycle characteristics are very poor.
Therefore, as a method to improve cycle characteristics, it has been considered to inhibit such expansion by alloying tin or silicon (Si). For example, it has been suggested that iron (Fe) and tin are alloyed (refer to “Journal of The Electrochemical Society,” 1999, Vol. 146, p. 414). Further, Mg2Si or the like has been suggested (for example, refer to “Journal of The Electrochemical Society,” 1999, Vol. 146, p. 4401).
However, even in the cases using the foregoing methods, it is actual situation that effects of improving cycle characteristics are not sufficient and advantages of the high capacity anode in the alloy material are not sufficiently utilized.